1. Field of the Invention
The present invention relates generally to the art of molding articles and more particularly to the elimination of flash during molding. In its most preferred embodiment, the present invention relates to a molding process and apparatus for flat panels which reduces flash by employing movable side plates which close prior to complete parison formation, blowing and mold closure.
2. Description of the Prior Art
Blow molding of plastic parts, such as flat panels used in outdoor sheds, tops for coolers and numerous other products has been accomplished by employing a mold which includes two mold halves. In one traditional molding process, a parison, or tube of hot plastic material is extruded downwardly between the mold halves. As the mold begins to close, the bottom of the parison is pinched closed to allow the plastic material to be blown to fill the mold cavity. In traditional molds the parison exceeds the dimensions of the mold at the time of final mold closure creating flash. Once heat is removed from the plastic, such as by the use of cooling water flowing through the mold halves, the part is removed, trimmed and further processed for its end use application.
While this blow molding process has been successfully used for many years, trimming is time consuming, labor intensive and therefore expensive. Moreover, a trim line is created around the part which may not yield a desirable appearance. In addition, a greater amount of material must be processed than would be necessary if the flash could be eliminated. Since the material itself is made by heating a plastic starting material to a molten condition, energy is wasted for that portion of the molded product which must be trimmed away. In addition, the part weight is typically higher using traditional molding than would be necessary if the flash were not produced, and the cycle time for a complete molding operation is high. Furthermore, the flash removal process can be relatively dangerous, and the elimination of the manual step of cutting plastic from a part could reduce lost time injuries, workers"" compensation claims and the like. The development of a reduced flash molding process which is adaptable to a wide variety of articles, including flat panels, would represent a significant advance in this art.
The present invention features a reduced flash molding process and apparatus which eliminates at least a part of the post molding trimming operation by addressing the source of the flash in the mold.
Another feature of the present invention is a reduced flash molding process and apparatus which eliminates excess flash along at least the sides of the molded article, reducing production cost and cycle time.
A different feature of the present invention is to provide a reduced flash molding process and apparatus which allows a part to be made using less material, thereby reducing part weight and reducing energy costs.
A still further feature of the present invention is to provide a reduced flash molding process and apparatus in which overall cycle time is reduced.
A still further feature of the present invention is to provide a reduced flash molding process and apparatus in which the mold can be placed closer to the plastic extrusion die, yielding greater control during parison formation and handling during the molding operation.
Yet another feature of the present invention is to provide a reduced flash molding process and apparatus which may be used for molding techniques other than blow molding of plastic parts, e.g. in glass molding, vacuum forming and the like.
How the foregoing features of the invention are accomplished individually, and in various combinations, will be described in detail in the following description of the preferred embodiment, taken in conjunction with the drawings. Generally, however, the reduced flash molding process and apparatus of the present invention includes the use of members at the sides of the mold which are closed before or while a parison is extruded (when using blow molding), following which the mold closes around the parison. While the mold closes, the parison is blocked by the side members which prevents any of the molding material from extending outside of the mold where the side members are located, and flash is formed only at the top and the bottom of the mold. Following formation of the part, trimming of only the top and bottom is required. Certain features are also provided by placing the extrusion die nearer the mold to improve parison management. As the parison begins to be extruded from the die, and since the extrusion gap is relatively small, parison swell can be enhanced to ensure better material distribution. As parison shooting progresses, the die gap itself is increased to give the parison sufficient strength to hold its own weight, and when the parison is fully extruded, the diameter at the die itself is smaller than the diameter at the bottom. Following prepinching, which can be done closer to the mold using the present invention, the parison is enlarged, such as by blowing air into it, so that the parison extends to and contacts the side members, which in turn prevent any material escape and allow a more uniform pre-blow inflation. The reduced flash molding process and apparatus of the present invention also permit the use of less material due to parison control at the top of the mold. Other ways in which the features of the invention may be individually or collectively accomplished will become apparent to those skilled in the art after they have read this specification. Such other ways are deemed to fall within the scope of the present invention if they fall within the scope of the claims which follow.